Diffusion furnaces have been widely used for thermal processing of semiconductor device materials (such as semiconductor wafers or other semiconductor substrates). The furnaces typically have a large thermal mass that provides a relatively uniform and stable temperature for processing. However, in order to achieve uniform results, it is necessary for the conditions in the furnace to reach thermal equilibrium after a batch of wafers is inserted into the furnace. Therefore, the heating time for wafers in a diffusion furnace is relatively long, typically exceeding ten minutes.
As integrated circuit dimensions have decreased, shorter thermal processing steps for some processes, such as rapid thermal anneal, are desirable to reduce the lateral diffusion of dopants and the associated broadening of feature dimensions. Thermal process duration may also be limited to reduce forward diffusion so the threshold voltage of the MOS transistors does not shift. As a result, the longer processing times inherent in conventional diffusion furnaces have become undesirable for many processes. In addition, increasingly stringent requirements for process control and repeatability have made batch processing undesirable for many applications. As an alternative to diffusion furnaces, single wafer rapid thermal processing (RTP) systems have been developed for rapidly heating and cooling wafers. Most RTP systems use high intensity lamps (usually tungsten-halogen lamps or arc lamps) to selectively heat a wafer within a cold wall, clear quartz furnace. Since the lamps have very low thermal mass, the wafer can be heated rapidly. Rapid wafer cooling is also easily achieved since the heat source may be turned off instantly without requiring a slow temperature ramp down. Lamp heating of the wafer minimizes the thermal mass effects of the process chamber and allows rapid real-time control over the wafer temperature. While single wafer, lamp-based RTP reactors provide enhanced process control, their throughput is substantially less than batch furnace systems.
While lamp-based RTP systems allow rapid heating and cooling, it is difficult to achieve repeatable, uniform wafer processing temperatures using them, particularly for larger wafers (200 mm and greater). The temperature uniformity is sensitive to the uniformity of the optical energy absorption as well as the radiative and convective heat losses of the wafer. Wafer temperature nonuniformities usually appear near wafer edges because radiative heat losses are greatest at the edges. During heating the wafer edges may, at times, be several degrees (or even tens of degrees) cooler than the center of the wafer. At high temperatures, generally greater than nine hundred degrees Celsius (900xc2x0 C.), this nonuniformity may produce crystal slip lines on the wafer (particularly near the edge). To minimize the formation of slip lines, insulating rings are often placed around the perimeter of the wafer to shield the wafer from the cold chamber walls. Otherwise, wafer temperature non-uniformity may lead to non-uniform material properties such as alloy content, grain size, and dopant concentration. These non-uniform material properties may degrade the circuitry and decrease yield even at low temperatures (generally less than 900xc2x0 C.). For instance, temperature uniformity is critical to the formation of titanium silicide by post deposition annealing. The proper alloy is formed only within a range of temperatures of several degrees. In fact, the uniformity of the sheet resistance of the resulting titanium silicide is regarded as a standard measure for evaluating temperature uniformity in RTP systems because it so sensitively reflects the precise temperature at which the silicide was formed.
Wafer temperature levels and uniformity must therefore be carefully monitored and controlled in lamp-based RTP systems. Optical pyrometry is typically used due to its noninvasive nature and relatively fast measurement speed which are critical in controlling the rapid heating and cooling in RTP. However, accurate temperature measurement of wafer temperature using optical pyrometry depends upon the accurate measurement of the intensity of radiation emitted from the wafer and upon the wafers radiation emitting characteristics or emissivity. Emissivity is typically wafer-dependent and depends on a range of parameters, including temperature, chamber reflectivity, the wafer material (including dopant concentration), surface roughness, and surface layers (including the type and thickness of sub-layers), and will change dynamically during processing as layers grow on the surface of the wafer. In addition, radiation from heat sources, particularly lamps, reflect off the wafer surface and interfere with optical pyrometry. This reflected radiation erroneously augments the measured intensity of radiation emitted from the wafer surface and results in inaccurate wafer temperature measurement. It is therefore preferable in the interest of reduced system complexity and cost of processing in an RTP system to have means for controlling the wafer temperature other than one requiring it to be accurately measured such as is required with the lamp-based systems. It is desirable from the standpoint of cost per wafer processed (avoidance of expensive wafer emissivity measurement apparatus) to be able to control the process by keeping the wafer temperature within the desired processing range without directly measuring the wafer temperature.
In order to overcome the disadvantages of lamp heated RTP systems, a few systems have been proposed which use a resistively heated plate. Such heated plates provide a relatively large thermal mass with a stable temperature. The inherent temperature uniformity of wafer heating by a plate (whose lateral thermal conductivity is high compared with that of the wafer) is superior to that of a wafer by itself, such as is characteristic of the common lamp-based RTP systems. Therefore, by employing such a plate or susceptor for wafer heating, where the temperature uniformity of a wafer on the block is measured and is made to be well within specifications prior to usage for device wafer processing, the wafer temperature can be made to be highly uniform during processing.
While heated plate rapid thermal processors provide a stable temperature on the heated plate that may be measured using a thermocouple, problems may be encountered with wafer temperature nonuniformities. Wafers may be heated by placing them near the heated plate rather than on the plate. In such systems, the edges of the wafer may have large heat losses which lead to nonuniformities as in lamp heated RTP systems. Even when a wafer is placed in contact with a heated plate, there may be nonuniformities. The heated plate itself may have large edge losses, because: 1) the corners and edges of the plate may radiate across a wider range of angles into the chamber; 2) vertical chimney effects may cause larger convective heat losses at the edges of the heated plate; and 3) the edges of the heated plate may be closer to cold chamber walls. These edge losses on the plate may, in turn, cause temperature nonuniformities in a wafer placed on the plate.
In addition, heat loss and temperature uniformity across the wafer surface varies with temperature and pressure. Conductive heat transfer between two objects (such as the wafer and the cold chamber wall) is proportional to the temperature difference between the objects and radiative heat transfer is proportional to the difference of the temperatures raised to the fourth power (T14xe2x88x92T24). Thus, the difference in heat losses across the wafer surface (resulting in wafer temperature differences) will increase at higher processing temperatures. In addition, the pressure in the chamber may affect the wafer temperature profile since heat transfer at low pressures is predominantly by radiation, while heat transfer at higher pressures involves a combination of radiation, conduction and convection.
Another important aspect of a thermal processing system is its ability to provide the same wafer temperature to a cassette of wafers regardless of the thicknesses and types of layers on the wafer backside. During processing wafers for the same process may have varying backside coating(s) most commonly of silicon, silicon dioxide or silicon nitride. These coatings will tend to cause variation in the emissivity of the wafer backside which will affect the heating rate of the wafer and its equilibrium temperature when radiation is an important part of the heat flow in the system. It is not uncommon for wafers with the same process needed on the front side to have different backside coatings or none at all. Thus, in order for a system to be commercially viable it must process such wafers in such a manner as to yield effectively the same wafer temperature regardless of the backside coatings. When the front of the wafer is exposed to a substantially cooler top or walls in a heated plate system, the wafer temperature may vary significantly with the backside emissivity whenever the radiative heat transfer is a significant part of the heat flow from the heater to the wafer. Were it possible to have the temperature of the wall nearer to that of the wafer there would be less dependence of the wafer temperature on the emissivity of the wafer backside.
Conventional heated plate processing systems also tend to be energy inefficient. The heated plate is maintained at a high temperature with constant conductive, convective and radiative losses to the cold chamber walls. While conductive and convective losses may be reduced at lower pressures, this also inhibits the heat transfer to the wafer. At low pressures where heating is primarily radiative, the wafer may be significantly cooler than the heated plate particularly when proximity heating is used. This makes the wafer temperature difficult to control. Further, at low pressures where radiation is the primary mechanism for heat transfer, the variance in wafer temperature uniformity across temperature ranges may be greater because heat transfer by radiation is proportional to the difference between surface temperatures raised to the fourth power (T14xe2x88x92T24). Thus, decreasing pressure to increase energy efficiency may make the wafer temperature and uniformity more difficult to control.
One aspect of the present invention provides a semiconductor substrate processing system and thermal processing method. The processing system comprises a base (or primary) resistive heater and an edge (or peripheral) resistive heater for heating one or more blocks having a large thermal mass, and insulating walls surrounding the heaters and the block(s) for insulating a thermal processing region. The system dimensions and processing parameters are preferably selected to provide a substantial heat flux to the substrate while reducing the potential for heat loss to the surrounding environment, particularly from the edges of the heated block and the substrate. The heat source (comprising the resistive heaters and the heated block) provides a wafer temperature uniformity profile that is substantially constant across a large temperature range at low chamber pressures.
In one aspect of the present invention, one or more semiconductor substrates may be simultaneously transferred from a storage cassette into a load lock; from the load lock into a processing chamber by passing the substrates through a port in a wall of the processing chamber; and thence onto support pins capable of raising and lowering the substrate(s) relative to the heated block. The heated block receives heat from dual resistive heaters comprising a primary or base resistive heater located below the heated block, and a peripheral or edge heater positioned substantially around the edges of the base heater. In one embodiment of the invention, the heaters may be fabricated from silicon carbide coated graphite. Temperature measuring sensors such as thermocouples and optical pyrometers may be inserted into or placed adjacent the heated block; in one aspect of the present invention, a thermocouple is arranged such that its tip opposes that of an optical pyrometer. The thermocouple may be used to calibrate the optical pyrometer.
Preferably, the heated block and the resistive heaters are substantially enclosed within insulating walls that form the thermally insulated cavity. It is an advantage of one aspect of the present invention that the insulated cavity be formed by multiple layers of these insulating walls, and the layers may be arranged in a substantially concentric manner. The innermost insulating walls on the top, bottom, and sides of the insulated cavity may comprise silicon carbide coated graphite, and the outer walls may comprise opaque quartz. In one aspect of the invention, there may be three layers of insulation at the top and bottom of the cavity, and two layers of insulation around the sides, thus forming a compact design. Insulating layers that form the bottom of the insulated cavity may be supported by posts that also serve to space apart the bottom insulating walls from the bottom chamber wall and the heater block. An insulating shutter may be moved adjacent to the entrance port to reduce heat loss from the thermally insulated cavity when substrates are not being transferred into and out of the chamber.
The dual heater system and multiple layers of insulation provide a high-level of thermal uniformity within the chamber, and this uniformity may be maintained across a wide range of temperatures.
An additional aspect of the present invention includes sleeves that surround the elevational pins which raise and lower the substrate relative to the heated block. The sleeves help to isolate the regions of the chamber containing the resistive heaters from the thermal cavity in which the substrates are processed, and serve to substantially prevent gases containing trace metals in the vapor phase (originating from the resistive heaters) from contaminating the substrates. The sleeves may comprise clear quartz, opaque quartz, silicon carbide, or some other ceramic.
An additional aspect of the present invention includes a vacuum spool or baffle inserted within the foreline of the exhaust system. The spool has a large conductance pathway for exhausting the region of the processing chamber containing the heaters, and a small conductance pathway for exhausting other regions of the chamber not containing heaters. Furthermore, the spool assists in protecting sensitive components of the exhaust system, such as sealing o-rings, by lowering the temperature of the exhaust gases before they enter the vacuum pump(s)l The spool also acts to reduce heat loss from the chamber.
Aspects of the present invention include methods of using the thermal processing system. One of the critical attributes of these methods is the pressure at which the system is operated, since pressure helps to determine the rate of the temperature ramp. A single processing step may be employed if the load lock pressure is the same as the processing pressure; a three step process may be used if the load lock pressure is different from the processing pressure. Typical load lock pressures for steps 1 and 3 are about 2 to 3 Torr, but may be as high as 100 Torr; exemplary processing pressures in step 2 range from 10 to 50 Torr, and any range subsumed therein.